1. Field of the Invention
The present invention relates to a back light unit having a light guide buffer plate, and more particularly, to a back light unit having a light guide buffer plate capable of improving image quality by improving visibility, uniformity and luminance since the light guide buffer plate is provided between a light source and a light guide panel to get rid of a poor-visibility zone without any changes in structure of the light guide panel.
2. Description of the Related Art
In general, flat displays are mainly divided into emissive flat displays and non-emissive flat displays. Examples of the non-emissive flat displays include a liquid crystal display, and it is impossible to see an image on such a liquid crystal display in dark places since the liquid crystal display does not emit the light by itself to display an image, but uses incident light from the external environment to display an image.
Accordingly, a back light unit is installed in the rear of the liquid crystal display to illuminate the liquid crystal display with light. The back light unit has been used for surface light source apparatuses such as an illuminating sign and the like in addition to the non-emissive display such a liquid crystal display.
The back light unit is divided into a direct light type and an edge light type, depending on the configuration of the light source, the direct light type using a plurality of lamps installed right beneath the liquid crystal display to directly illuminate a liquid crystal panel, and the edge light type using a lamp installed in an edge of a light guide panel (LGP) to illuminate a liquid crystal panel with light.
The edge light-type back light unit uses a line light source and a point light source as the light source. Representative examples of the line light source include a cold cathode fluorescent lamp (CCFL) having both end electrodes installed in the inner part of a tube, and the point light source includes a light emitting diode (LED).
Here, the CCFL has advantages that it may emit intensive white light, provide high luminance and high uniformity, and make it possible to design a large back light unit. However, it has disadvantages that it is operated by a high-frequency AC signal and within the narrow temperature range. The LED has advantages that, although its performance is deteriorated in the aspect of luminance and uniformity when compared to the CCFL, it is operated by a DC signal, its life span is long, and its operation temperature range is wide. Also, the LED has advantages that it may be manufactured with a thin thickness.
And, the light guide panel used for the edge light type back light unit functions to convert incident light into a surface light and release the surface light in a vertical direction, the incident light being transmitted from a line light source or a point light source through an edge of the light guide panel. To convert incident light from the light source into a surface light, a scattering pattern or a holographic pattern is formed on the light guide panel using a printing or mechanical machining process.
FIG. 1 is a configurational view illustrating a conventional back light unit.
The conventional edge light type back light unit 1 includes a light source 10 formed as line light source or a point light source in a substrate 12; a light guide panel 20 disposed in one side of the light source to convert light into a surface light and release the converted light source toward a liquid crystal panel (not shown); and a reflecting member 30 disposed below the light source 10 and light guide panel 20 to reflect the light released from the light source, as shown in FIG. 1.
A pattern 25 is provided in a bottom surface of the light guide panel 20 facing the reflecting member 30, the pattern 25 being convexly or concavely formed to convert light into a surface light.
On the light guide panel 20 is formed an optical sheet 40 such as a diffuser sheet or a prism sheet, wherein the diffuser sheet diffuses light in various directions, the light being reflected by the reflecting member 30 and refracted in the light guide panel 20 to direct toward the liquid crystal panel (not shown), and the prism sheet plays a role in collecting the light, passed through the diffuser sheet, within the range of a front viewing angle.
For the edge light type back light unit 1 as configured thus, the light from the light source 10 is released toward an arrow A direction represented by a solid line to enter a light incidence surface 21 which is one side surface of the light guide panel 20. Then, some of the incident light from the light incidence surface 21 passes through the light guide panel 20 as shown in an arrow A1 direction, some of the incident light collides against the pattern 25 of the light guide panel 20 and the reflecting member 30 and then directs toward the optical sheet 40 as shown in an arrow A2 direction, and the remainder of the incident light collides against the optical sheet 40 and then reflects toward the pattern 25 and the reflecting member 30 as shown in an arrow A3 direction.
However, the edge light type back light unit 1 has disadvantages that the incident light converted in a vertical direction is ununiformly scattered in an end region of the light guide panel 20 that is adjacent to the light source, its luminance is poor, and a poor-visibility zone (W) in which colors are not normally mixed is formed. Therefore, the poorly mixed colors serve to deteriorate user's visibility since they are seen as spots in the display panel.
Therefore, optical luminance and uniformity are additionally deteriorated even when a light source is enhanced, a pattern processed especially in a bottom surface of the light guide panel is improved, or a structure of the light guide panel is changed for the purpose of solving the above problem regarding the visibility zone.